Shani Wallis, TunnelTalk
- Another tunnel collapse has hit the headlines this week. This time it is in Prague on a highway tunnel being built on a ring road around the Czech Republic's capital. Local newspaper reports tell of a collapse crater on the surface of 20m wide x 30m long and of an excavator driver and his machine being buried under the fall, but rescued and released after a short hospital stay. Fortunately, there are no reports of serious injuries or fatalities or of structural damage to surface buildings or underground infrastructure in the area. The collapse is said to have occurred in the small hours of Tuesday 6th July at 1am, and on a section of tunnel designed and being constructed according to the NATM concept.
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Heathrow Express collapse in 1994
- This collapse adds to a list of 'NATM' collapses which is matched, if not surpassed, by a list of TBM sinkholes and collapses of underground structures associated to open-box failures, slurry wall or ground freezing breaches, or accidents of other kinds.
- The mechanisms of collapses can be investigated in minute detail, as many have been - for example the court case enquiry of Heathrow Express collapse in London, the public investigation of the open station box collapse at Nichol Highway in Singapore, the failure of ground freezing cross-passage excavation in Shanghai - or they can be categorized using broad criteria. The reports of fine detail investigations are available for review and there are studies on collating collapse events and listing prime causes but for the purposes of this discussion let us make our own list.
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For NATM collapses, the questions for inquiry can begin with:
• Is it really a NATM (or SME (sequential method excavation) or SCL (sprayed concrete lining) design? Are the fundamentals of the concept part of the design and construction process? Is the shape of the tunnel and are the cycles of the excavation sequences (topheading, bench, invert, sidewall drifts) appropriate for the prevailing ground conditions? Flat inverts and inverts not closed to form rings of adequate support are not going to succeed in soft ground. Failure to apply the necessary support at the necessary time will cause trouble. Collapse of highway tunnel in Albania can be scrutinised under these criteria.
• When did the collapse occur? The collapse in Prague, like nearly all others, occurred at night, on the night shift implying two things. Lack of construction work supervision and lack of engineering staff on hand to take command if something irregular is noticed or initiated. Nobody wants to work the nightshift and the B team is often assigned to the nightshift.
• What is the real NATM experience of the designer and of the contractor organisations - and of the engineers, managers and tunnelling crews put in charge of the works? There can be no excuse for inexperience of designers or contractors, the consequences are too grave, and the tunnelling crews, whether experienced or not, are in their charge.
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Lining mistake caused the TBM metro drive sinkhole in Cairo
- For TBM sinkholes, the principal cause is over-excavation - frequently recorded with an inexperienced operator at the helm and often occurring again during the night shift. There have been other famous TBM tunnel cases caused with nothing to do with the TBM or its operation. Examples include the recent Cairo Metro collapse where a segment fell out of a ring of segmental lining already built because, allegedly and on good authority, without the bolts inserted, and the case in Los Angeles where the method implemented to correct a metro tunnel alignment error lead to a collapse hole in the middle of Hollywood Boulevard.
- Other collapses are the result of alleged criminal acts, or the consequential result of earlier risk-analysis decision making or of directions given to construction crews without referring back to the design adequacy of works already in the ground. The recent failure of the open-cut crossover structure on the new metro line in Cologne, Germany that occurred in the middle of the day, caused catastrophic destruction of adjacent city buildings and claimed the lives of two citizens, can be cited as an example. The most recent suggested cause of the collapse is fraud and corruption among the managers of the construction team who were selling off materials that should have been built into the works. Less criminally indicting causes include inadequacy of the dewatering support systems, retaining walls that were not deep enough to prevent the invert of the box from boiling, failure to insert support anchors or tiebacks, and excavation of a sump in the middle of the box to a depth below which the perimeter walls as designed could support.
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Courtesy Der Spiegel Catastrophic collapse of metro open cut work in the city of Cologne
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Learning from mistakes
Last week, on our Alert message from the Editor's Desk, we suggested that learning from mistakes is a dangerous precedent. That it requires that mistakes must be made in the first instance and that there are few lessons to be learned without them. Better to learn, we suggested, from successes, since repeating successes is productive and progressive. The mistakes of others, it seems, are rarely good teachers and you need to make mistakes first to learn from your own. - The collapse this week in Prague appears to support that argument. For sure the mistakes that led to the collapse in Prague are not made on this occasion for the first time. The same mistakes have been made before and they proved unable to prevent this repeat. Having learned the lesson themselves, it is hoped that the designers, managers and crews involved will know how to avoid making the same mistakes in future but there are many hundreds - thousands even - of kilometres of successful NATM tunnels built around the world from which better, more progressive lessons could have been learned.
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Heathrow failures highlight NATM misunderstandings
Tunnel collapse in Albania closes highway - TunnelTalk, Nov 2009
Sinkhole bothers Brightwater - TunnelTalk, March 2009
Cairo Metro tunnel collapse - TunnelTalk, Sept 2009
Fatal collapse on Cologne's new metro line - TunnelTalk, March 2009
The insurance consequences of failures - TunnelTalk, Aug 2010
- Extrenal news report of alleged corruption in reference to the Cologne tunnel collapse.
Carlo Bretz, Switzerland
Interesting report. One could add a long list of other collapses like Glendoe and Gigel Gibe II etc.
I agree with you: one should learn from success not mistakes. Confucius says: "Experience is like a lantern in your back. It lights only the already traveled path!"
If you look at the top sports trainers, they never tell of the mistakes you have made. Only of what you have done well and how to improve on it!
Carlo Bretz, Switzerland
Rock falls shut down Glendoe power plant - TunnelTalk, Aug 2009
Glendoe rockfalls more serious than initial fears - TunnelTalk, Oct 2009
Recovery contract for failed headrace at Glendoe - TunnelTalk, Feb 2010
Collapse of headrace tunnel after grand opening - TunnelTalk, Feb 2010
Repair of limited collapse in Ethiopia - TunnelTalk, Mar 2010
From a reader in Germany
Big problems for the industry, in my personal opinion, are price dumping, budget cutting, and time saving - and that in a field of highest safety requirements. Tunnelling industry has sometime not too much respect for its own achievements, and should not let advocates make the business. Most importantly on the negative said is talking badly about competition and about the industry itself.
From a reader in Germany
Charles W. Egerton, North Ayrshire, UK
One factor that you touch on is the fact that supervision at night tends to be lessened and that the construction team is often the B team. It has been observed on many occasions that junior newly qualified engineers on the night shift are placed with an experienced team headed by a foreman or supervisor who is usually older and in theory should have more experience. Whilst many young engineers see things being done that they consider as being wrong, they cannot comment for the reason that they will be referred to as being 'wet behind the ears' and by being told that "this is the way it has to be done". Another syndrome that takes place on the night shift is that the absence of watching eyes results in the cutting of corners.
Any senior engineer on the night shift will be working alone and will have been left with a list of functions related to sorting out the back-up infrastructure and completing unfinished paper work.
There are two things that should be dispensed with:
a) Advance rate bonuses, which have the effect of making it almost inevitable that requirements in other areas will be forgotten. An example is quiet simply blocked annular grouting pipes resulting in not being able to keep grouting at the same speed as TBM advance, but carrying on with excavation regardless.
b) 12 hour shift patterns, which lead to workers, in an already stressed environment, being over tired and taking their minds off the ball. Cutting down shift times is an area that meets with considerable hostility. It is time for projects to be managed by the engineers (managers) who must face the flack of any incident, rather than finding someone lower down the chain of command to take the blame.
You also mention the failure in Cairo, in which you mention that bolts were "forgotten". Were they forgotten or did someone come to the conclusion that bolts are to be removed later, so why put them in the first place! People forget the rotational forces applied by the TBM. Cutterhead torque has to go somewhere. As Newton's Third Law that states: "For every action there is an equal and opposite reaction." I have observed personally segment joints opening up as a result of cutterhead torque. In some instances this has sheared both dowels and bolts and opening up the longitudinal segment joints.
Charles W. Egerton, North Ayrshire, UK
From a reader in the USA
I think you might want to modify slightly your comments on learning from failures to include the following:
All of us should be lucky enough to start in our 20s learning from our own small failures. More is learned from bad jobs than good jobs. Perhaps as important is to pay attention to details of mistakes that "almost" or "could have" resulted in very serious consequences.
I believe that most of the serious cave ins you write about would not have occurred had a couple of the people in charge (including designers, owners, engineers, contractors, etc) had more 'failure' experiences in their 20s. Unfortunately that kind of good experience is less common now and perhaps the reason why we are seeing more of these unnecessary problems.
Thank you for writing about this topic in your editorials. It needs more knowledgeable discussion.
A reader in the USA
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